Cetane Number is a measure of the ignition performance of a diesel fuel compared to the ignition performance of reference fuels in a standardized engine test. Within the context of evaluating the Cetane Number of a fuel in accordance with ASTM D613-10a (“Standard Test Method for Cetane Number of Diesel Fuel Oil”), ignition performance refers to the ignition delay of the fuel as determined in a standard test engine under controlled conditions of fuel flow rate, injection timing and compression ratio. The Cetane Number is currently defined with respect to two primary reference fuels n-cetane and heptamethylnonane. N-cetane is assigned a Cetane Number of 100, and heptamethylnonane is assigned a Cetane Number of 15. Volumetrically proportioned mixtures are assigned a Cetane Number equal to the volume percent of n-cetane added to the volume percent of heptamethylnonane multiplied by 0.15.
The Cetane Number of a test sample is determined by comparing its combustion characteristics in a test engine with those for blends of reference fuels of known Cetane Number under standard operating conditions using a bracketing hand-wheel procedure which varies the compression ratio (hand-wheel reading) for the test sample and each of two bracketing reference fuels to obtain a specific ignition delay by interpolation of Cetane Number in terms of hand-wheel reading.
A complete description of the test apparatus, reagents, reference materials, sampling, engine and instrument settings, standard operating conditions, calibration and engine qualification, and test procedure is provided in ASTM D613-10a.
The performance of a diesel engine is greatly impacted by the combustibility of the diesel fuel (light distillates oil) used. Similar to the octane number rating that is applied to gasoline to rate its ignition stability, Cetane Number is the rating assigned to diesel fuel to rate its combustion quality. For gasoline octane number signifies its ability to resist auto-ignition (also referred to as pre-ignition, knocking, pinging, or detonation). For diesel Cetane Number is a measure of the fuel's delay of ignition time (the amount of time between the injection of fuel into the combustion chamber and the actual start of combustion of the fuel charge).
Since diesels rely on compression ignition (no spark), the fuel must be able to auto-ignite—and generally, the quicker the better. A higher Cetane Number means a shorter ignition delay time and more complete combustion of the fuel charge in the combustion chamber. This, of course, translates into a smoother running, better performing engine with more power and fewer harmful emissions.
A fuel with a low Cetane Number can cause a diesel engine to operate rough and generate higher emissions as the fuel is not burned as efficiently as it might with a higher Cetane Number fuel. Low Cetane Number fuels also make it more difficult to start the engine, while a diesel fuel with a high Cetane Number will ignite more readily, burn more completely, and deliver more power than fuels with lower numbers. In other words, Cetane Number (CN) is a measure of a fuel's ignition delay which is the time period between the start of injection and the first identifiable pressure increase due to combustion of the fuel.
Historically, accurate measurements of the Cetane Number have been difficult. It requires burning the fuel in a rare diesel engine called a Cooperative Fuel Research (CFR) engine, under standard test conditions (ASTM D613 method). The operator of the CFR engine, which was first developed in 1932, uses a hand-wheel to increase the compression ratio (and therefore the peak pressure within the cylinder) of the engine until the time between fuel injection and ignition is 2.407 ms. The resulting Cetane Number is then calculated by determining which mixture of known Cetane Number Primary Standard fuel will result in the same ignition delay.
Two Primary Reference Fuels (hydrocarbons) define the Cetane Number scale and all diesel fuel and diesel fuel components are indexed to the Primary Reference Fuels. Initially, n-hexadecane (also called n-cetane or n-C16H34), which has very good ignition quality, was assigned the Cetane Number of 100, and 1-methylnaphthalene, which has a poor ignition quality, was assigned a Cetane Number of zero.
The industry had problems in producing 1-methylnaphthalene with consistent high quality and in 1962, this low Cetane Number Reference Fuel standard was replaced with 2,2,4,4,6,8,8-heptamethylnonane (also called isocetane or HMN). This chemical had better oxidation stability and was easier to use in the CFR engine. When measured against the original 1-methylnaphthalene and n-cetane standards, 2,2,4,4,6,8,8-heptamethylnonane was assigned a Cetane Number of 15.
Thus, when a fuel has the same ignition delay period as a mixture of the two Primary Reference Fuels (n-cetane+HMN), its Cetane Number is derived from the volume percent of Cetane and HMN, as follows:Cetane Number=% n-cetane+0.15(% HMN)=15+0.85(% n-cetane)
It has been difficult to produce pure HMN in large quantities, of the right purity, at reasonable cost. Therefore, in routine operations, the two Primary Reference Fuels (n-cetane and HMN) are replaced by two Secondary Reference Fuels: T-fuel and U-fuel. Large batches of Secondary Reference Fuel pairs are calibrated against the Primary Reference Fuels and made available to testing labs. The fuel supplier provides blend ratio instructions to achieve Cetane Numbers bounded by the values for the U and T fuels.
Each set consists of a “T Fuel” having a relatively high Cetane Number (typically 72-76) and a “U Fuel” which has a relatively low Cetane Number (typically 18-20). The Cetane Numbers assigned to each batch are based on a testing program conducted by the Diesel NEG in which blends of the T and U Fuels are engine tested against the ASTM D613-10a Primary Reference Fuels (PRF) n-cetane and 2,2,4,4,6,8,8-heptamethylnonane (HMN).
Seven Secondary Reference Fuels (T and U fuel) are engine tested (ASTM D613) by various labs as follows:
Blend No.% T% U12080240603505046040570306802071000
Each blend of T and U fuels is tested by bracketing the blend against the known Cetane Number blend of Primary Reference Fuels n-cetane and HMN. A regression equation for Cetane Number as a function of % T Fuel is then developed. Typical correlations for the pairs of T and U Secondary Reference Fuel batches developed by the industry over the years are listed below:T-24/U-17(2007)Cetane Number=0.554513(% T)+19.50861T-23/U-16(2002)Cetane Number=0.568440(% T)+19.25631T-22/U-15(1997)Cetane Number=0.560351(% T)+18.72438T-21/U-14(1994)Cetane Number=0.554019(% T)+18.27033T-20/U-13(1990)Cetane Number=0.527952(% T)+19.92739
These equations were then used to generate the tables of accepted reference value Cetane Numbers for various blends of T and U fuel pairs as shown, typically, in Table A.
TABLE ACETANE NUMBER for BLENDS OF T-24 AND U-17% T-24CN019.5120.1220.6321.2421.7522.3622.8723.4823.9924.51025.11125.61226.21326.71427.31527.81628.41728.91829.51930.02030.62131.22231.72332.32432.82533.42633.92734.52835.02935.63036.13136.73237.33337.83438.43538.93639.53740.03840.63941.14041.74142.24242.84343.44443.94544.54645.04745.64846.14946.75047.25147.85248.35348.95449.55550.05650.65751.15851.75952.26052.86153.36253.96354.46455.06555.66656.16756.76857.26957.87058.37158.97259.47360.07460.57561.17661.77762.27862.87963.38063.98164.48265.08365.58466.18566.68667.28767.88868.38968.99069.49170.09270.59371.19471.69572.29672.79773.39873.99974.410075.0
The diesel fuel industry today continues to be burdened by the complex program of producing and validating T and U Secondary Fuel pair batches every one or two years for use in measurement of Cetane Numbers of diesel fuels and diesel components using the CFR engines and ASTM test method D613-10a.
As mentioned earlier, the low Cetane Number Primary Reference Standard, HMN of the 98% minimum purity level required is not currently available in large quantities. Also, the very small laboratory quantities that are available are cost prohibitive. The diesel fuels industry has therefore resorted to using Secondary Reference T and U Fuels. Ideally, the industry requires a substitute for HMN that can be produced in large quantities at reasonable cost so that the HMN substitute can be used not only to standardize the T and U Secondary Fuel blends, but the CFR engines of the ASTM D613 could be standardized to utilize blends of Primary Reference Fuels, n-cetane and a HMN replacement.
In addition to the CFR engine test as specified in ASTM D613, there are other alternatives to describe ignition quality. These tests include Cetane Index, which is calculated from other fuel properties such as density and volatility, and Derived Cetane Number (DCN) calculated from the ignition delay time measured using a constant volume combustion chamber method. Cetane Index is based on the fact that ignition quality is linked to hydrocarbon composition: n-paraffins have high ignition quality, and aromatic and naphthenic compounds have low ignition quality. Cetane Index being a calculated value, only provides an approximate indication of actual ignition quality.
The DCN is determined by measuring ignition delay in a constant chamber method. One method that has emerged is a combustion-based analytical method that was originally developed at Southwest Research Institute (ASTM D6890). It is referred to as the Constant Volume Combustion Apparatus (CVCA). A commercial apparatus utilizing this technique, introduced by Advanced Engine Technologies, is known as the Ignition Quality Tester (IQT). In this test, a small specimen of diesel fuel is injected into the heated, temperature controlled constant volume chamber which has previously been charged with compressed air. Ignition delay is measured using sensors that detect the start of fuel injection and the start of combustion for each cycle. Calibration of the apparatus is carried out with two reference materials: n-heptane and methylcyclohexane. The heptane is assigned an average ignition delay of 3.78±0.01 ms while the methylcyclohexane is assigned an ignition delay of 10.4±0.6 ms. The DCN is calculated using an equation that correlates a combustion ignition delay result to actual Cetane Number measured in the engine (ASTM D613).
Another test based on Ignition Delay to determine the DCN is ASTM D7170. The instrument to perform the DCN test here is offered by Waukesha and is known as the Fuel Ignition Tester (FIT). FIT also uses n-heptane and methylcyclohexane for calibration.
Yet another method for DCN is the Cetane ID (CID) 510 by PAC. This method is outlined in ASTM D7668 and uses the Ignition Delay measurement in a CVCA and correlates it to Cetane Number measured in an engine (ASTM D613).
An objective of certain aspects of this disclosure involves replacing mixtures of n-cetane and HMN as Primary Reference Fuels for calibration of the CFR engines (ASTM D613) and the calibration of the IQT (ASTM D6890), FIT (ASTM D7170) and CID (ASTM D7668) with mixtures of n-cetane and a lower cost, more easily available compound that HMN, but which has a Cetane Number that is nearly the same as that of HMN, and which produces blends having performance characteristics similar or substantially the same as HMN/n-cetane blends.